The present invention relates to a control system for the air conditioning plants of a building and, more particularly, to a control system to minimize the energy consumed by the transport and chiller systems of a building in delivering cool air to the building zones.
Typical commercial and other large present day buildings or building complexes comprise a plurality of floors each floor having exterior zones having at least one wall exposed to the outdoors and interior zones having no walls exposed to the outdoors. A zone is defined as a room or group of rooms or an area of a floor or floors. Each floor of the building may have a plurality of fan systems for delivering treated air to the interior and exterior zones depending upon the size of the floor or floors of the building. For example, the interior zones of a building represent a cooling load both in winter and summer whereas the exterior zones of a building represent a cooling load in summer and a heating load in winter. Thus, it is typical to connect the interior zones to one fan system and the exterior zones to a second fan system. Of course, if the size of the floor is sufficient, the interior and exterior zones may each be supplied by more than one fan system.
There have been a number of efforts to minimize the energy consumed by the air conditioning systems of a building. For example, the prior art has devised load cycling systems for cycling on and off fans, radiators and other energy consuming equipment in air conditioning systems, outdoor air damper control systems for most effectively utilizing outdoor air, load shedding when energy consumption in the building approaches a peak limit, and the like. All of these prior art systems have substantially reduced the energy consumed by present day buildings.
The present invention increases these energy savings by recognizing that there is a tradeoff between the amount of energy required to transport cooled air to the building zones and the amount of energy consumed by the chiller in chilling the water supplied to the cooling coils of the fan systems to a sufficient degree to satisfy the needs of the zones connected to the fan system. The transport energy may be defined simply as the energy required by the fan system to deliver cool air to the zones or may be defined as a combination of the fan energy plus the pump energy expended by the pump associated with the chiller in supplying chilled water from the chiller to the cooling coils of the fan systems. Thus, at a particular building load it may be cheaper to use more fan rather than more chiller whereas at a different load it may be cheaper to use more chiller and less fan. These savings cannot be recognized by those systems which merely select a chilled water supply temperature and adjust a fan accordingly. According to the present invention, savings can be increased by selecting a chilled water supply temperature and a cooperating fan speed which will satisfy the zone conditions at a minimum expenditure of energy.